Implement heuristic to prioritize reflection info in field descriptor search

[augusto2112]

Currently when looking for field descriptors we parse the reflection
metadata in whatever order it was registered. This patch implements a
heuristic where we try to match a new optional Name field with the
module name of the type's field descriptor we're looking for.

rdar://87889973

[augusto2112]

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[slavapestov]

The mangled type name already includes the module name. Is this for correctness or performance?

[augusto2112]

@slavapestov performance. In lldb's case (not sure about the runtime) it can be quite expensive to normalize thousands of mangled names before finding the one we're looking for

[augusto2112]

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[slavapestov]

it can be quite expensive to normalize thousands of mangled names before finding the one we're looking for

What does 'normalize' mean? Perhaps when loading the image we should build a hashtable or something instead? It seems unnecessary to add redundant information to the field descriptor.

[augusto2112]

What does 'normalize' mean?

Essentially converting a mangled name with symbolic references, substitutions, etc, into a "normal" mangled name (look at normalizeReflectionName).

Perhaps when loading the image we should build a hashtable or something instead?

Building the hashtable of MangledName -> FieldDescriptor is the slow part 😆 The problem is that currently there's no way to know in what image the field descriptor is, so we parse (and cache) everything in whatever order the images were registered. If the field descriptor we're looking for happens to be near the end of that list, we parse everything else in front of it. In the app I'm profiling we normalize upwards of 20k mangled names (which takes 5+ seconds when debugging on a real device) when we're looking for only two field descriptors.

It seems unnecessary to add redundant information to the field descriptor.

We're only adding one fairly short StringRef per image (ReflectionInfo), we're not actually touching the field descriptors themselves. This shouldn't be too bad.

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[kastiglione]

both calls to PopulateFieldTypeInfoCacheWithReflectionInfo are followed by this lookup. Should there be some function, say FindFieldDescriptor(Info, *MangledName), which searches and also handles the population/caching as an implementation detail?

[jckarter]

Would it make sense for the Name to be a SmallVector of names, in case there are types from multiple modules present in an image?

[augusto2112]

@jckarter yes I think that makes a lot of sense.

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[augusto2112]

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[augusto2112]

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